A common usage for a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is as a switch. In such applications, MOSFETs function as a switch between two terminals, the drain and the source, when the proper control voltage is applied to the gate terminal. For applications requiring high current and medium voltage commercially available MOSFETs are of the N-channel, enhancement mode type. The enhancement mode MOSFET requires that the gate be more positive than the source by some voltage, called the enhancement voltage, in order to open the channel between the drain and the source. The enhancement voltage required to turn on commercially available MOSFETs is typically called the threshold voltage and is usually in the range of one to four volts.
Prior art designs of MOSFET switch control circuits, wherein the control is isolated from the switching MOSFETs, cannot effectively switch high frequency ac signals (herein called the "input signal") at high speed, high power, and high switching frequencies. One problem encountered in some designs is a distortion of the input signal due to modulation of the input signal by the enhancement voltage signal applied to the gate. Another problem in devices which employ an oscillator to drive the MOSFET gate occurs because the input signal may pass through the isolation device (transformer, capacitor, etc.) used between the oscillator and the switch. This pass through effect becomes more likely as the ratio of the oscillation frequency approaches the frequency of the switched signal and may result in serious damage to the circuit.
Similar devices are also subject to distortion of the input signal due to switching transients. These transients are present in MOSFETs due to capacitance in the gate to source junction. During the operation of turning a MOSFET switch on or off, the gate to source junction appears as a capacitor. In steady state operation, it is only necessary to maintain this charge against very small leakage currents. For high-speed switching, a large current must be passed to the gate and this may result in distortion to the input signal.
FIG. 1 shows an example of a MOSFET switching circuit using two enhancement mode MOSFETs and an isolated transformer drive. In the circuit shown in FIG. 1, the gate voltage at node 10 is referenced directly to the source voltage at node 20. Such a design causes a current to flow into the source. When the oscillator 50 is enabled, the oscillator-generated current will mix with the input signal current. When the oscillator is disabled, any current present in the circuit will flow, or divide and flow, into the circuits connected to both sides of the transistor switch depending on the voltage levels present. This type of current flow is caused by the internal diodes 30 and 40 (shown in dotted line) that are a part of the internal structure of all enhancement mode MOSFETs. A circuit such as is shown in FIG. 1 has size and cost disadvantages. In addition, at higher switching frequencies, some of the oscillator frequency signal will mix with and modulate the input signal. This type of modulation can also result if the magnitude of the input signal is very small in comparison to the oscillation signal. As the oscillator frequency approaches the frequency of the input signal, major distortion of the input signal can take place.